/Engine/Plugins/Runtime/MassGameplay/Source/MassSignals/Public/MassSignalProcessorBase.h
To receive notification about a particular signal, you need to subscribe to it.
void SubscribeToSignal ( [UMassSignalSubsystem](API\Plugins\MassSignals\UMassSignalSubsystem) & SignalSubsystem, const [FName](API\Runtime\Core\UObject\FName) SignalName )